Identifying sources of non-determinism
Fundamentally, real-time programming is about making sure that the threads controlling the output in real time are scheduled when needed and so can complete the job before the deadline. Anything that prevents this is a problem. Here are some problem areas:
- Scheduling: Real-time threads must be scheduled before others, and so they must have a real-time policy,
SCHED_FIFOorSCHED_RR. Additionally, they should have priorities assigned in descending order, starting with the one with the shortest deadline, according to the theory of rate monotonic analysis that I described in Chapter 17. - Scheduling latency: The kernel must be able to reschedule as soon as an event such as an interrupt or timer occurs and not be subject to unbounded delays. Reducing scheduling latency is a key topic later on in this chapter.
- Priority inversion: This is a consequence of priority-based scheduling, which leads to unbounded delays when a high-priority...
